A high electron mobility transistor (HEMT) has a hetero-junction structure formed by stacking a low-temperature buffer layer made of GaN, a buffer layer made of GaN, an electron transit layer made of GaN, and an electron supply layer made of AlGaN in this order on a substrate. Further, the HEMT has a source electrode, a gate electrode, and a drain electrode on the electron supply layer.
In the HEMT, the electron supply layer has a large band gap energy, compared with the electron transit layer, and a two-dimensional (2D) electron gas layer is formed below a hetero-junction interface between the electron transit layer and the electron supply layer. The 2D electron gas layer is used as a carrier. That is, when the source electrode and the drain electrode are turned on, electrons supplied to the electron transit layer move in the 2D electron gas layer at a high speed to reach the drain electrode. At this time, by changing a thickness of a depletion layer under the gate electrode by controlling a voltage applied to the gate electrode, it is possible to control the electrons moving from the source electrode to the drain electrode, i.e., a drain current.
Regarding the HEMT, it has been generally considered that an electric field strength distribution between a gate and a drain during an off time is uniform. However, the electric field strength on the drain side may be maximized depending on the structure of the HEMT, leading to a possibility that the drain side reaches a dielectric breakdown. In particular, this phenomenon stands out as a drain voltage is increased.
Further, such a problem is not limited to the HEMT, and for example, may also arise between an anode and a cathode of a Schottky barrier diode. In this case, the dielectric breakdown may occur on the cathode side.